Outside the Northern Mountain Shrine, the forest was silent. Moonlight spilled through the tangled branches, scattered into fragments across the ground and over the two figures standing opposite each other.
Iguro Obanai held his Nichirin Sword tightly, his sharp eyes glinting with tension. The faint purple aura of Serpent Breathing coiled along his blade like a venomous snake ready to strike.
Facing him, Tomioka Giyu stood motionless, the hem of his black kimono fluttering slightly in the night wind. His dark blue slit pupils were calm and unreadable. "Become a demon, Iguro."
A bitter laugh escaped Obanai's lips. "Heh… I'll never become a monster like you."
With a burst of motion, he lunged forward, blade twisting unpredictably like a serpent's tongue. "Serpent Breathing, Third Form: Coil Choke!"
Giyu didn't move. The blue moon pattern on his Nichirin Sword flared to life as he parried the strike with effortless precision. "Moon Breathing, Second Form: Pearl Flower Moongazing!"
Three arcs of pale blue light swept through the air. Obanai's sword was deflected, and a shallow wound appeared on his arm.
Black blood seeped from the cut, his arm going numb as his grip weakened.
"Your attacks are flexible," Giyu said flatly, his tone distant and heavy with dominance. "But you lack strength. Against a demon's true power, human breathing techniques are meaningless."
He moved again. Moon Breathing's flowing slashes surged like the tide, crashing forward. Each strike avoided Obanai's vital points but still left new wounds in its wake.
Obanai fought back with everything he had. The speed of his Serpent Breathing increased, but no matter how he twisted or countered, he couldn't breach Giyu's defense.
He knew this was only an act—but even so, he could feel the overwhelming pressure of Giyu's strength. If this were a real fight, he thought grimly, I wouldn't last ten moves.
"Kh—!"
The back of Giyu's blade slammed into Obanai's spine. He coughed blood, his body collapsing forward, his Nichirin Sword clattering to the floor.
Giyu stepped closer, pressing the cold edge of his blade against the back of Obanai's neck.
"Still want to fight?" Giyu's voice was quiet, emotionless. "Become a demon, and you'll gain endless life. You'll be stronger than ever. Free from the limits of a human body. Don't you want power? Don't you want to exist forever?"
Obanai trembled slightly, as though wavering.
He stayed silent for a moment, then slowly raised his head, his eyes filled with reluctant defiance. "Fine. I'll do it. I'll become a demon."
Giyu withdrew his sword, a faint glimmer of satisfaction flickering in his dark pupils. "Good."
He crouched down and gathered a drop of black blood on his fingertip, holding it out. "Drink this, and the transformation will begin. Remember—from this moment on, your life no longer belongs to you. It belongs to Lord Muzan. No betrayal will be tolerated."
Obanai stared at the dark blood for a long second before finally opening his mouth and swallowing it.
Immediately, black mist erupted from within him, wrapping his entire body. His bones cracked audibly, his veins bulged, and pale serpent scales began to form faintly beneath his skin.
It was only the initial stage of transformation—to become an Upper Moon, Muzan himself would still need to grant his own blood.
"Let's go," Giyu said, straightening. He turned toward the direction of Infinity Castle. "I'll take you to meet Lord Muzan."
Obanai staggered to his feet, pain surging through his changing body. The demonic power raging inside him was intoxicating and terrifying at once—his strength multiplied several times over, but along with it came a foreign presence creeping into his consciousness.
It was Muzan's will, pressing against his mind like a whisper in the dark.
Clenching his teeth hard enough to draw blood, Obanai fought to stay conscious, forcing that invasive will back through sheer discipline and unyielding loyalty to the Demon Slayer Corps.
And with that resolve burning faintly within him, he followed Giyu into the depths of the night.
…
At the heart of Infinity Castle, darkness swallowed everything. Kibutsuji Muzan sat upon his throne in silence, his crimson eyes narrowing as they swept across the two figures entering the grand hall.
When his gaze fell on Iguro Obanai, his pupils contracted slightly. Within Obanai's mismatched eyes, beneath the demonic ferocity, there burned a deep and bitter hatred. Muzan could see it clearly—the kind of hatred that fueled strength. It pleased him.
"This is the one you recommended, Mushiki?"
Muzan's voice was calm but edged with scrutiny.
Giyu bowed deeply. "Yes, Lord Muzan. He possesses remarkable talent and unwavering restraint. He also harbors resentment toward the Demon Slayer Corps. If granted your blood, he will surely become a worthy Upper Moon Six."
Obanai lowered his head, his voice hoarse and obedient. "Your servant, Iguro Obanai, greets Lord Muzan. I vow to serve with loyalty and destroy every last member of the Demon Slayer Corps!"
He deliberately let his tone carry that false "hatred," deepening the illusion to earn Muzan's trust.
A thin smile crossed Muzan's lips as black mist gathered in his palm. "Excellent. I like demons with obsessions. The stronger your obsession, the greater your power."
He stepped forward, lowering his hand until his clawed fingers pierced Obanai's back.
Thick, black blood poured through his veins, far denser and stronger than the trace Giyu had given earlier.
Obanai's body shuddered violently as immense energy flooded through him. The searing pain faded, replaced by a rush of overwhelming vitality. The faint serpent-like scales beneath his skin became clearer, and letters began to appear within his dual-colored eyes:
"Upper Moon Six."
"From this moment, you are Upper Moon Six," Muzan said coldly. "Your name will be Yatsuki."
Retracting his claws, Muzan's tone softened with faint approval. "Doma's territory and his unfinished tasks now belong to you."
"I obey, Lord Muzan."
Obanai bowed low, his voice laced with forced reverence.
Satisfied, Muzan turned his gaze toward Giyu. "You've done well. Remain in the Entertainment District and continue to observe the Demon Slayer Corps' movements. Assist Yatsuki in understanding his duties as an Upper Moon. Do not disappoint me."
"Yes, Lord Muzan," Giyu replied, bowing again.
Muzan's form dissolved into the shadows, leaving the hall silent once more.
The two stood there for a long moment before stepping out into the endless corridors of Infinity Castle.
Only when Giyu was certain that no other demonic presence lingered nearby did he stop walking and speak quietly, his voice just above a whisper. "Being a demon doesn't feel great, does it?"
Obanai turned his head slightly, expression blank. "Feels disgusting, honestly."
Giyu gave a faint nod. "Doma's territory is unstable. Don't act too soon. Watch carefully first—avoid drawing attention."
"Understood."
Obanai's tone was clipped but steady. The two said nothing more.
They parted ways in silence—Giyu heading back toward the Entertainment District to continue his role as Mushiki, while Obanai moved south to seize control of Doma's old territory and begin his covert mission.
In the eternal darkness of Infinity Castle, their figures drifted farther apart.
A knife and a serpent—both hidden at Muzan's side.
And with the birth of the new Upper Moon Six, the dangerous game of infiltration and deception grew even more intricate, drawing all of them closer to the final goal.